(1) Field of the Invention
The present invention relates to a blood backscattering power obtaining apparatus used in ultrasonography. Recently, in the field of ultrasonography, the blood backscattering power has been used as a reference value for evaluating the intensity of a backscattering power of a tissue.
(2) Description of the Related Art
It is known that the backscattering power of a portion of a tissue in a human body provides useful information on a morbid state of the portion of the tissue. For, example, a tissue of a heart suffering from myocardial or cardiac infarction gives a higher value of the backscattering power compared with a normal tissue of a heart, and a fatty liver also gives a higher value of the backscattering power compared with a normal tissue of a liver.
Intensities of an ultrasound signal propagated from an ultrasonic transducer to an objective portion of a tissue, and an ultrasound signal generated by blood backscattering, propagated to the ultrasonic transducer, are attenuated with their propagation lengths, and the degree of the attenuation depends on the characterization of the tissues through which the ultrasound signals propagate.
Since it is known that the backscattering power of blood does not essentially vary between different persons, and a large blood vessel or a portion containing blood usually exists near a tissue of which the blood backscattering power is to be measured, it is proposed to use the backscattering power of blood in a large blood vessel or a body containing blood, e.g., the heart or other organ, near a tissue of which a blood backscattering power is to be measured, as a reference value for evaluating the degree of the attenuation which an ultrasound signal is suffered while the ultrasound signal is propagated from an ultrasonic transducer to the tissue, is backscattered by the tissue, and is propagate back to the ultrasonic transducer i.e., evaluating the backscattering power of the tissue. This technique is proposed by K. Nakayama and S. Yagi in "In Vivo Tissue Characterization Using Blood Flow Doppler Signal as a Reference" in Japanese Journal of Medical Ultrasonics, Vol. 15, Supplement I, 1988 (Jun. 4, 1988, in Japanese), by S. Miyagi et al. in the Technical Report of the Institute of Electronics, Information and Communication Engineers, Vol. 88, No. 307, US-88-47 (Dec. 1, 1988, in Japanese), by the Unexamined Japanese Patent Publication No. 1-297052 (Dec. 30, 1989), and by the U.S. Pat. No. 4,867,167 granted to P. A. Magnin on Sep. 19, 1989. Although the blood backscattering power varies with a hematocrit value of blood (a volume ratio of blood cells in blood), the hematocrit value can be measured, and therefore, a blood backscattering power of an individual person can be corrected by his hematocrit value.
The outline of the above technique is schematically indicated in FIG. 1. In FIG. 1, reference numeral 1 denotes an ultrasonic transducer, 2 denotes an ultrasonic beam, 4 denotes a vessel which is filled with blood, 5 denotes a portion of the tissue which is to be examined, and 6 denotes the position in the blood vessel at which a value of the blood backscattering power is obtained. The above position in the vessel which is filled with blood, may be the interior a blood vessel, or a ventricle or an atrium cordis of a heart.
However, it is known that ultrasound signals which are generated by the blood backscattering are detected by the ultrasonic transducer together with strong clutter components. FIG. 2 shows several examples of causes of the clutter components. In FIG. 2, reference numeral 1 denotes the a ultrasonic transducer, 11 denotes wall of a blood vessel, 12 denotes inside of the blood vessel, 13 denotes a main beam of an ultrasound signal, 14 denotes a position at which the blood backscattering power is to be obtained, 15, 17, 19, and 21 each denote a side lobe beam of the main beam of the ultrasound signal, 16 and 18 each denote a portion of the wall of the blood vessel located at a position of the same propagation delay D, i.e., distance from the ultrasonic transducer as the position 14 of the blood backscattering, and 20 and 22 each denote a portion of a tissue not of interest, but which is located at a position of the same propagation delay D from the ultrasonic transducer as the position 14 of the blood backscattering. As understood from FIG. 2, ultrasound signals which have been reflected at boundaries of different tissues 16, 18, or backscattered in some tissues 20 and 22 which are not of interest, reach the ultrasonic transducer 1 at the same time as ultrasound signals which have been generated by the blood backscattering in the vessel to be examined. Namely, the ultrasound signals which have been reflected at boundaries of different tissues or backscattered in some tissues which are not of interest, are superimposed on the ultrasound signals which have been generated by the blood backscattering. It is known that the intensities of the ultrasound signals which are generated by the blood backscattering are smaller than the intensities of the clutter components, the ultrasound signals which have been reflected at boundaries of different tissues or backscattered in some tissues which are not of interest, by about 30 to 50 dB.
Therefore, it is necessary to effectively remove the clutter components, when obtaining the blood backscattering power, from the above ultrasound signals which include the blood backscattering component and the clutter components.